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Calendar
Calendar A calendar is a system of organising days for a socially, religious, commercially or administratively useful purpose. This organisation is done by giving names to periods of time - typically days, weeks, months and years. The name given to each day is known as a date. Periods in a calendar (such as years and months) are usually, though not necessarily, synchronised with the cycles of some astronomical phenomenon, such as the cycle of the sun, or the moon.
Many civilisations and societies have devised a calendar, usually derived from other calendars on which they model their systems, suited to their particular needs.
A calendar is also a physical device (often paper). This is the most common usage of the word. Other similar types of calendars can include computerised systems, which can be set to remind the user of upcoming events and appointments.
As a subset, "calendar" is also used to denote a list of particular set of planned events (for example, court calendar).
Calendar systems A full calendar system has a different calendar date for every day. Thus the week cycle is by itself not a full calendar system; neither is a system to name the days within a year without a system for identifying the years.
The simplest calendar system just counts days from a reference day. This applies for the Julian day. Virtually the only possible variation is using a different reference day, in particular one less distant in the past to make the numbers smaller. Computations in these systems are just a matter of addition and subtraction.
Solar calendars Days used by solar calendars Solar calendars assign a date to each solar day. A day may consist of the period between sunrise and sunset, with a following period of night, or it may be a period between successive events such as two sunsets. The length of the interval between two such successive events may be allowed to vary slightly during the year, or it may be averaged into a mean solar day. Other types of calendar may also use a solar day.
Lunar calendars Not all calendars use the solar year as a unit. A lunar calendar is one in which days are numbered within each lunar phase cycle. Because the length of the lunar month is not an even fraction of the length of the tropical year, a purely lunar calendar quickly drifts against the seasons. It does, however, stay constant with respect to other phenomena, notably tides. A lunisolar calendar is a lunar calendar that compensates by adding an extra month as needed to realign the months with the seasons. An example is the Jewish calendar which uses a 19 year cycle. Lunar calendars are believed to be the oldest calendars invented by mankind. Cro-Magnon people are claimed to have invented one around 32,000 BC.
Calendars with Easter Sunday Calculating the calendar of a previous year is a relatively easy matter when Easter Sunday is not included on the calendar. However, calculating for Easter Sunday is a tricky piece of business since the calculation requires the knowledge of the full moon cycle. Easter Sunday is on the first Sunday after the first full moon after the Vernal Equinox according to the computus. So, this makes an additional calculation necessary on top of the normal calculation for January 1st and the calculation of whether or not the year is a leap year.
There are only 14 different calendars when Easter Sunday is not involved. Each calendar is determined by the day of the week January 1st falls on and whether or not the year is a leap year. However, when Easter Sunday is included there are quite a few different calendars.
Fiscal calendars A fiscal calendar (such as a 5/4/4 calendar) fixes each month at a specific number of weeks to facilitate comparisons from month to month and year to year. January always has exactly 5 weeks (Sunday through Saturday), February has 4 weeks, March has 4 weeks, etc. Note that this calendar will normally need to add a 53rd week to every 5th or 6th year, which might be added to December or might not be, depending on how the organization uses those dates. There exists an international standard way to do this (the ISO week). The ISO week starts on a Monday, and ends on a Sunday. Week 1 is always the week that contains 4 January in the Gregorian calendar.
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Calendar
2008
Calendar
2008 Calendar - Year
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